add gcc-4.5 for compiling the kernel

copied mostly from openembedded spring 2012, slightly adopted
bitbake target is "gcc-cross-kernel"
add TUNE_CCARGS which allow the kernel to boot
linux-su980: use kernelgcc to build
linux-su980: remove accidentally commited hack
switch kernel image type to "Image" instead of "vmlinux"
gcc-cross-kernel: only install gcc-4.5.4
gcc-cross-kernel: hardwire $TARGET-as

(about the same as commit 9947882a in meta-tripledragon)
linux-su980: fix KERNEL_CC definition
su980-dvb-modules: remove bogus dependencies
update defconfig-su980 kernel config

* add devtmpfs (needed by udev)
* build-in ext3 FS, so that it can be used as rootfs
* increase dmesg buffer
su980.conf: build ext3 images as default, too